<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body {
            background-color: lightgreen;
        }
        
        @media screen and (max-width: 600px) {
            body {
                background-color: pink;
            }
        }
    </style>
</head>

<body>
    <div class="sky">
        重置浏览器查看大小。当浏览器窗口的宽度小于 300 像素时，背景颜色会变成淡蓝，否则是淡绿色
    </div>
</body>

</html>